Paleocene-Eocene Thermal Maximum (PETM)

The PETM was a period of rapid climate change triggered by sudden changes in …

The Paleocene-Eocene Thermal Maximum was an instance of geologically sudden climate change that occurred about 55 million years ago. Over the span of approximately 10,000 years, the global temperature appears to have risen approximately 5-8°C. The rapid change was accompanied by major disruptions in the planet's ecosystem. Temperatures remained elevated for roughly 100,000 years, before returning to levels typical of that period.

The changes in temperature appear to have been caused by the addition of massive amounts of greenhouse gasses to the atmosphere. The ratio of carbon-12 to carbon-13 in the atmosphere experienced a rapid shift, as large amounts of isotopically light carbon was released from an unknown source. Since light isotopes are preferentially used by living matter, candidates for the source include methane clathrates and organic-rich sediments, both of which are generated by microbial activity.

This carbon was almost certainly in the form of methane and/or carbon dioxide, which accounts for the sudden warming via greenhouse influences (methane in the atmosphere rapidly becomes oxidized to carbon dioxide, so the ultimate result is elevated CO2). The elevated carbon dioxide also produced ocean acidification, lowering the pH as it is dissolved. This triggered a significant extinction event in the oceans.

Terrestrial ecosystems did not suffer as much, but were severely altered by the sudden rise in temperature. As with modern climate change, growing zones shifted towards the poles; plants that had previously been found near the Gulf Coast of North America became common near the present US-Canadian border during the PETM.

The trigger for the PETM and the precise amount of carbon released remain uncertain.